The former head of Russia’s Federal Security Service (FSB) has said that spies follow an “unwritten law” against poisoning the relatives of rival agents and cast doubt on accusations that Moscow had attempted to kill a former double agent in Britain.

Sergei Skripal, a former intelligence officer who served jail time in Russia for spying for Britain, was poisoned in southern England with his daughter on March 4. British Prime Minister Theresa May said on Monday that it was “highly likely” that Russia was behind the Skripal’s poisoning.

“The operative’s family has immunity because if you start acting through the wife, the children, this process will become irreversible, and it can turn into a virtual blood feud,” he said, citing unwritten intelligence agency rules.

The ex-FSB chief went on to say that Skripal had posed no threat to Russia, allowing him to be part of a spy exchange in 2010 for Russian agents held in the United States.

“After an evaluation, it was determined that Skripal posed no threat to Russia,” Kovalyov said.